SailNet Community banner

1 - 20 of 33 Posts

·
Master Mariner
Joined
·
8,221 Posts
Discussion Starter #1
If a generator produces 4.4 kilowatts @ 238 volts, 60 Hz, will it produce 8 kW @ 120 volts, 60 Hz?
I was having a discussion with an electrician today who says not. He also said that the Onan 8 kW Genset rated @ 8 kW driven by a 10.5 hp diesel engine cannot possibly produce more than 6 kW.
All MDKD Onan gensets are rated as 8 kW units @ 1800 rpm and it seems as though there must be something wrong with either this gentleman's math or Onan's rating of 8 kW.
Any thoughts?
 

·
Registered
Joined
·
198 Posts
If a generator produces 4.4 kilowatts @ 238 volts, 60 Hz, will it produce 8 kW @ 120 volts, 60 Hz?
I was having a discussion with an electrician today who says not. He also said that the Onan 8 kW Genset rated @ 8 kW driven by a 10.5 hp diesel engine cannot possibly produce more than 6 kW.
All MDKD Onan gensets are rated as 8 kW units @ 1800 rpm and it seems as though there must be something wrong with either this gentleman's math or Onan's rating of 8 kW.
Any thoughts?
HP and wattage are both measures of power, one electrical and one physical but can be directly converted.

Your electrician friend is correct 1 hp is 745.69 watts so at 10.5 hp the max wattage it can produce is 7829.745 that is assuming 100% conversion of power between mechanical power, hp, and electrical power, kw. Onan is over stating the generators output by rounding to the nearest thousand, at 10.5 hp the maximum it could produce, without loses and assuming 100% conversion is the above stated 7829.745 watts. No diesel generator can convert 100% efficiently from mechanical power to electrical, so in my opinion this is just Onan overstating the engines ability if it is true. When I did a check of Onan generators online the 8 KW models had 22.3 hp engines attached.

By the way for your first example, assuming the generator is capable of multiple voltages it would produce 4.4 KW, half the voltage but twice the amperage, as wattage = voltage X amperage.
 

·
Registered
Joined
·
7,056 Posts
On the current boat first had 4 kw lombardini. It was a total piece of junk. When I replaced it an new 8 kw Northern Lights was available at a extremely good price so installed that.
I never maxed out the output of the 4 kw unit. The current genset goes on rarely (has 17hours since Oct of 2018). When it goes on I charge batteries, vacuum, turn on the AC, watermaker, charge all electronics and tools etc. but obviously still don’t max out the 8kw unit.
I knew I wanted a low rpm unit. Smallest I could find was a 6kw and it was considerably more expensive than the 8kw.
Did I make a mistake in buying the 8kw? How important is it that a genset be run under full load occasionally?
 

·
Registered
Joined
·
21,630 Posts
I'm not sure, but I doubt it's a problem to run a genset below full load. They run at constant rpm anyway. I suspect it may be poor for the diesel to run it at zero load for lengthy periods of time, but no one would do that. Curious too.
 

·
Registered
Joined
·
198 Posts

·
Registered
Joined
·
3,711 Posts
I always understood genset wattage ratings to be their max spike load capacity, but continuous load was notably lower.
Our 8kW Kohler genset has a 14hp engine and is continuously rated for 8kW at 77F. It derates 1% for every 10F above 77F.

Our 5.5kW Nextgen genset has a 11hp engine and is continuously rated for 5.0kW at 77F. I don't know its derate factor for temperature, but know that it doesn't like running higher than ~4.5kW at typical hotter temps.

Mark
 

·
Registered
Joined
·
21,630 Posts
Found this https://boatmags.com/choosing-the-right-generator/ interesting and informative read.
That was a good read. This was the answer and it would seem unlikely many genset run with less than 25% load or one probably wouldn't be running them at all. I got a chuckle out of the recommendation that one consider having two different size generators to properly match loads.

A generator should never run continually with less than a 25% load. 35% to 70% is optimal.
 

·
Registered
Joined
·
7,056 Posts
So...
We never run the generator for just one load. Doing best guesses we’re at just above the 35% during the usual hour or so the time the thing is on. In retrospect the 6kw would’ve been enough but the 8kw should be alright.
Thanks for the help.
 

·
Registered
Joined
·
59 Posts
It is very common for generators to have shortened lives because of either low engine loading or just lack of use at all. There have been a lot of heated discussions on the TrawlerForum regarding minimum loading of engines. One of the biggest factors is whether the engine is one of the new Tier-4 electronically controlled common rails, or just an old fashioned mechanical engine.

Assuming mechanical, I would estimate 35% being a safe minimum and you'd probably want to run it once a week for several hours. Certainly higher loading is better, but unless you're running your aircon, it may be difficult to provide such a high load. Perhaps timing water heater, battery bank bulk charge, electric oven and clothes washer/dryer.

I had two gennies in my power boat and hated both of them. My opinion is that unless you have aircon, then do not mount a genset. Today's energy efficient lighting and appliances, coupled with solar options, are all wonderful, whether for a sailboat or a powerboat.

BTW, outbound commented about installing a larger unit because the price was better, which is understandable. If you find yourself only running 2-3 kWhr over the next year, then consider having the injector pump (or other systems) de-tuned by a mechanic to a lower hp rating. The electrical head end does not care what it is loaded to (as long as the RPMs are correct), it's the diesel unit that needs to run properly.
 

·
Registered
Joined
·
198 Posts
BTW, outbound commented about installing a larger unit because the price was better, which is understandable. If you find yourself only running 2-3 kWhr over the next year, then consider having the injector pump (or other systems) de-tuned by a mechanic to a lower hp rating. The electrical head end does not care what it is loaded to (as long as the RPMs are correct), it's the diesel unit that needs to run properly.
Actually de rating the hp will could the output of the generator as less hp means less mechanical power to convert to electrical power, so de rating the diesel could de rate the output of the generator if the diesel does not generate sufficient power to produce the wattage, i.e if it where a 1hp engine it could produce 745.69 watts maximum assuming 100% conversion efficiency which they are not, if it were de rated to 0.8 hp a 20% reduction then the maximum electrical power that could be produced is 596.55 watts. In the end power is power whether it is Horse Power or electrical and changing one will result in changing the other.
 

·
Registered
Joined
·
21,630 Posts
A great point was made above that all engines need to run or rot. When conserving fuel on a passage, there is nothing wrong with avoiding consumption for days at a time. However, when fuel is available, the engines should be run to full temp and under load routinely. I don't have any science to support it, but I'd say that weekly would be an absolute minimum.
 

·
Registered
Joined
·
21,630 Posts
If one has the place for it, another great multi-tasking use for the genset is a washing machine. When cruising, our Splendide is a luxury beyond words. We do not bother drying (which uses more water than washing), but cleaning up the clothing with real soap and fresh water is amazing. This is never done stand alone, only when using the gen for all other tasks too. I'd say the gen burns a few gallons per week. It's pretty inconsequential.
 

·
bell ringer
Joined
·
4,761 Posts
Assuming mechanical, I would estimate 35% being a safe minimum and you'd probably want to run it once a week for several hours.
Several hours?????

Far as I'm concerned all you need to do is run it till it's fully warmed up and fully loaded for 10 minutes or so. I'm not going to listen to the generator for several hours just to exercise the unit. I also only shot for every couple of weeks.

Of course it will be summer soon, running it wouldn't be a problem. :laugh
 

·
Master Mariner
Joined
·
8,221 Posts
Discussion Starter #17
It is very common for generators to have shortened lives because of either low engine loading or just lack of use at all.
Assuming mechanical, I would estimate 35% being a safe minimum and you'd probably want to run it once a week for several hours. Certainly higher loading is better, but unless you're running your aircon, it may be difficult to provide such a high load. Perhaps timing water heater, battery bank bulk charge, electric oven and clothes washer/dryer.
I guess I neglected to mention that our last genset was a running take out at 19,000 hours plus. Certainly not without some problems at the end like rusting out freeze plugs and the alloy water pump housing corroding, but none the less a running take out.
As for operating our genset, it gets at least an hour in the morning and evening 365, with a water heater, high amp battery charger, fridge/freezer, and various kitchen appliances during cooking. When not cooking, the 35 gph watermaker goes on with its 17 amp draw, so I'm guessing a light load or little use won't be the death of this one.
In the marine field, I believe electronics are a big mistake in both gensets and main engines. Spare pc boards deteriorate at nearly the same rate as those in use and they are expensive. A good old mechanically regulated genset and main engine are worth their weight in gold if one is beyond the reach of Sea Tow or speedy delivery (1 day overnight usually takes about a week here).
 

·
Registered
Joined
·
21,630 Posts
Cooking has been the time when I'm most likely to run the genset. I refuse to crank it up, the moment I awake. A quiet cup of coffee is life extending. When breakfast or dinner prep starts, on comes the gen, but I like to shut it off to actually eat and relax. Watch the sunset peacefully. Evening becomes the toughest to schedule. Sometimes, sundowner cocktails delay dinner prep and time runs out. :)
 

·
Master Mariner
Joined
·
8,221 Posts
Discussion Starter #19
Cooking has been the time when I'm most likely to run the genset. I refuse to crank it up, the moment I awake. A quiet cup of coffee is life extending. When breakfast or dinner prep starts, on comes the gen, but I like to shut it off to actually eat and relax. Watch the sunset peacefully. Evening becomes the toughest to schedule. Sometimes, sundowner cocktails delay dinner prep and time runs out. :)
With Onan's sound shield, the genset is nearly silent, so it's nothing more than a background hum. Now that we've switched to gas cooking, it's mainly about refrigeration, battery charging, kitchen appliances to facilitate the cook's job and making and heating water.
Now the big question for us is switching from an electric coffee maker to a stovetop method. Any suggestions on which you like best? Please remember we are way south of civilization, so some suggestions won't be of much use to us.
My favorite the last time I had stovetop made coffee is pictured below, but if there is something better these days, I'm all ears.
 

Attachments

·
Registered
Joined
·
3,711 Posts
No reason to give up your electric coffee maker. If you have decent batteries and a suitable inverter, just run it off the inverter. They draw ~750W for maybe 5-7 minutes, so your batteries won't be hit hard. You will lose maybe 7-9Ah from your batteries.

Mark
 
1 - 20 of 33 Posts
Top